1. Field of the Invention
The invention relates to a motor driving circuit, and more particularly to a motor driving circuit for a step motor.
2. Description of the Related Art
Typically, in a conventional step motor, an H bridge circuit drives the step motor to rotate in different directions and also to stop. As shown in FIG. 1, an H bridge circuit 1 comprises four metal oxide semiconductors (MOSs). The direction of rotation of a motor 15 is controlled by turning on the first MOS 11 and the third MOS 13 or turning on the second MOS 12 and the fourth MOS 14. Input of voltage is required at the MOS gates to turn each MOS on or off. Because a different voltage is required for turning on each MOS, a different output voltage circuit controls each MOS. Thus, the gate of each MOS is coupled to an output voltage. Voltages from the output voltage circuits control the H bridge circuit to drive a step motor.
Each output voltage circuit has an input voltage. If the input voltage is used to directly turn on a MOS, excessive power is consumed, degrading efficiency. An output voltage circuit is thus required to raise an input voltage to a sufficient level to turn on a MOS, thereby driving the H bridge circuit to operate.
Because a conventional H bridge circuit comprises four MOSs, a gate of each MOS is coupled to an output voltage circuit, and each MOS is turned on by a high level voltage output from the output voltage circuit; thus, a motor is driven. When one set of two MOS, or a MOS pair, is turned on, the other two MOS pairs are turned off. If the MOS pair to be turned off is not actually turned off, or if the output voltages are unstable due to leakage current, the H bridge circuit may operate incorrectly and in the wrong directions, and the motor may be damaged. Thus, an output voltage circuit capable of ameliorating the described disadvantages is desirable.